Implement a helper for converting &str to CString. (#154)

This commit implements a simple helper for converting `&str` to `CString` and
mapping to the appropriate WASI error.

It also adds a `path_cstring` helper method in `PathGet` where the conversion was
used the most.

Fixes #104.

src/sys/windows/hostcalls_impl/misc.rs

@@ -14,7 +14,6 @@ lazy_static! {
     static ref START_MONOTONIC: Instant = Instant::now();
 }
 
-
 pub(crate) fn clock_res_get(clock_id: host::__wasi_clockid_t) -> Result<host::__wasi_timestamp_t> {
     unimplemented!("clock_res_get")
 }